1. Field of the Invention
This invention relates to measuring shot peening coverage by impact dent characterization. Such methods of this type, generally, employ a non-destructive, automated computer analysis by analyzing dent characteristics to determine shot coverage of a particular shot peened surface.
2. Description of the Related Art
Under normal shot peening conditions, it is desirable to expose the substrate or work surface to the shot stream for a sufficient time to achieve 100% surface coverage, i.e. sufficient exposure to fully cover the surface with impact dimples. Insufficient coverage has obvious adverse consequences. If the local compressive zones due to individual shot strikes do not merge into a continuum that fully covers the surface with a compressive layer, locally unprotected regions will exist where surface flaws can grow into cracks.
On the other hand, shot peening related damage is not necessarily caused only by broken or deformed shot, but rather excessive cold work may play a significant part in the damage process. The cold work sustained by the material is a function of both coverage and intensity, as well as, shot size. To minimize damage due to excessive cold work, or prevent overly worked parts from entering service, it is desirable to measure the coverage. Furthermore, for uniform flat surfaces it is relatively easy to set and control the exposure time needed to maintain the desired coverage condition. However, on surfaces with features, such as inside corners where reflections cause multiple strikes, it may be difficult to avoid excessive coverage, in some cases to the point where surface distress may occur.
Therefore it is apparent that two key drivers govern the shot peening process. They are intensity, or the impact energy of individual shot particles and coverage, or the way individual strikes add to fully cover the surface with a compressive residual stress layer. What makes coverage so important is that inadequate coverage can lead to gaps in the surface compressive layer, while excessive coverage or repeated beating can cause surface distress and cracking.
It is known, in prior non-destructive evaluation techniques (NDE), that a ratio of the total area of shot impact dents to the area of the peened surface can be used to detect coverage. Exemplary of such prior art is entitled "Effect of Partial-Coverage Upon the Fatigue Fracture Behavior of Peened Components" to S. A. Meguid, Fatigue Fract. Engng. Mater. Struct., Vol. 14, No. 5, pp. 515-530, 1991. By its very nature, this method is limited to measuring only up to 100% coverage or the time when 100% of the surface has been impacted. Therefore, a more advantageous system, then, would be presented if the system were based on the total number of strikes which would be valid for overlapping dimples and consequently sensitive to coverage percentages well above 100%, say to 1000%.
It is apparent from the above that there is a need in the art for a shot peening coverage system which detects the amount of coverage of the shot peening operation and which detects the amount of shot peening coverage in a non-destructive manner, but which at the same time is capable of detecting the amount of shot peening coverage well above 100% coverage. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.